Carbon dioxide adsorbent and carbon dioxide processing system

ABSTRACT

A carbon dioxide adsorbent including silica gel and an amine compound carried by the silica gel. The silica gel has a spherical shape, a particle size ranging from 1 mm to 5 mm inclusive, an average pore diameter ranging from 10 nm to 100 nm inclusive, a pore volume ranging from 0.1 cm3/g to 1.3 cm3/g inclusive, and a waterproof property N that is defined by an expression (1) and that is not lower than 45%,N=(W/W0)×100  (1)whereN is the waterproof property in percentage (%) of the silica gel,W0 is a total number of particles of the silica gel immersed in water,W is a number of particles of the silica gel not subjected to breakage out of W0.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Division of application Ser. No. 16/085,044 filed Sep. 14,2018, which in turn is a National Stage Entry of PCT/JP2017/010143 filedMar. 14, 2017, which claims priority to JP 2016-049103 filed Mar. 14,2016. The disclosure of each of the prior applications is herebyincorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a carbon dioxide adsorbent thatreversibly adsorbs carbon dioxide contained in a gas to be processed,and a method for manufacturing the same, and a system using the carbondioxide adsorbent.

BACKGROUND ART

A system configured to separate and remove carbon dioxide from a gas tobe processed containing carbon dioxide by using a solid carbon dioxideadsorbent is known in the related art. The carbon dioxide is included ina process gas discharged from a combustion facility such as a boiler.PTL 1 and PTL 2 disclose carbon dioxide separation systems of this kind.

A carbon dioxide separation system (carbon dioxide removal system)disclosed in PTL 1 includes a container accommodating a carbon dioxideadsorbent, and is configured to reversibly adsorb carbon dioxide from agas to be processed introduced into a container by using a solid carbondioxide adsorbent. The carbon dioxide adsorbent includes amine, a carbondioxide activating catalyst, and a porous substance configured to carrythe amine and the catalyst.

The system in PTL 1 performs processing based on a “batch processingmethod” that repeats a processing cycle including an “adsorbing step”for adsorbing and removing carbon dioxide from a gas to be processed byusing the carbon dioxide adsorbent and a “desorbing step” for desorbingthe adsorbed carbon dioxide from the carbon dioxide adsorbent. Incontrast, the system of PTL 2 described below performs the processingbased on a “continuous processing method” that performs the adsorbingstep and the desorbing step continuously in parallel.

The carbon dioxide separation system disclosed in PTL 2 includes ahopper, an adsorption tower where the adsorbing step is performed, adesorption tower (recovery tower) where the desorbing step is performed,a dryer tower where the adsorbent is dried, a cooling tower where theadsorbent is cooled, which are arranged downward in sequence in avertical direction, and a conveyer configured to transfer the adsorbentfrom the cooling tower to the hopper. The carbon dioxide adsorbentaccommodated in the hopper moves in sequence from the adsorption tower,the desorption tower, the dryer tower, and the cooling tower under itsown weight, and is transported from the cooling tower into the hopper bythe conveyer. In the adsorption tower and the desorption tower, a movingbed is defined by the carbon dioxide adsorbent moving downward in thetower and a gas moving upward in the tower. In the system of PTL 2, thecarbon dioxide adsorbent is a porous substance carrying an aminecompound, and the porous substance includes, for example, active carbonand active alumina.

CITATION LIST Patent Literature

PTL 1: JP 2012-501831 A

PTL 2: JP 2013-121562 A

SUMMARY OF INVENTION Technical Problem

In the system disclosed in PTL 1, the carbon dioxide adsorbent remainsstill with respect to the container. In contrast, in the systemdisclosed in PTL 2, the carbon dioxide adsorbent moves with respect tothe container, and thus friction or collision occurs between the carbondioxide adsorbent and the container, and between the carbon dioxideadsorbents. Therefore, specifically in the continuously processingmethod, a higher strength (in particular, abrasion resistance) isrequired more than the carbon dioxide adsorbent used in the batchprocessing method.

In view of such circumstances, it is an object of the present inventionto provide a carbon dioxide adsorbent having a superior adsorptiveproperty and a strength adapted to resist the usage in a continuousprocessing method, a method for manufacturing the same, and a systemusing the carbon dioxide adsorbent.

Solution to Problem

The present invention provides a method for manufacturing a carbondioxide adsorbent including:

preparing an amine aqueous solution having an amine compoundconcentration ranging from 5% to 70% inclusive and a temperature rangingfrom 10° C. to 100° C. inclusive;

impregnating silica gel with the amine aqueous solution; and

aeration-drying the silica gel carrying the amine compound, in which

the silica gel has a particle size ranging from 1 mm to 5 mm inclusive,an average pore diameter ranging from 10 nm to 100 nm inclusive, and apore volume ranging from 0.1 cm³/g to 1.3 cm³/g inclusive. However, allthe above-described particle size, the pore volume, and the average porediameter represent values of silica gel without carrying the aminecompound.

The carbon dioxide adsorbent of the present invention includes silicagel and an amine compound carried by the silica gel, the silica gelhaving a particle size ranging from 1 mm to 5 mm inclusive, an averagepore diameter ranging from 10 nm to 100 nm inclusive, and a pore volumeranging from 0.1 cm³/g to 1.3 cm³/g inclusive.

A carbon dioxide processing system according to the present inventionincludes an adsorption vessel including a moving bed, the moving bedbeing formed in an interior of the adsorption vessel by the carbondioxide adsorbent. A gas-to-be-processed supply port is formed at alower portion of the adsorption vessel and configured to receive asupply of a gas to be processed containing carbon dioxide, and anoff-gas discharge port is formed at an upper portion of the adsorptionvessel and configured to discharge an off gas, the off gas being a gasgenerated as a result of removal of carbon dioxide through adsorption tothe carbon dioxide adsorbent.

The carbon dioxide adsorbent and a method for manufacturing the sameprovide a carbon dioxide adsorbent being superior in carbon dioxideadsorbing performance and having abrasion resistance adapted to endureusage in the carbon dioxide processing system employing the continuousprocessing method.

Advantageous Effects of Invention

The present invention provides a carbon dioxide adsorbent having asuperior adsorptive property and a sufficient strength adapted to resistthe usage in a continuous processing method, a method for manufacturingthe same, and a system using the carbon dioxide adsorbent.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a schematic configuration of a carbondioxide processing system adapted to use a carbon dioxide adsorbentaccording to the present invention.

FIG. 2A is a view schematically illustrating a particle size of silicagel.

FIG. 2B is a view schematically illustrating a pore volume and a porediameter of the silica gel.

FIG. 3 is graph illustrating a relationship between carbon dioxide-airdiffusion resistance in a porous substance and the pore diameter.

FIG. 4 is a graph illustrating a relationship between the pore volume ofsilica gel in an adsorbent composed of silica gel carryingdiethanolamine and an amount of adsorption of carbon dioxide.

FIG. 5 is a graph of a result of a rotation abrasion test conducted onsilica gel.

DESCRIPTION OF EMBODIMENTS

A carbon dioxide adsorbent according to the present invention(hereinafter, referred to simply as an “adsorbent”) is used forreversibly adsorbing and removing carbon dioxide from a gas to beprocessed containing carbon dioxide. The adsorbent has a superior carbondioxide adsorbing performance and superior abrasion resistance and issuitable to be used in a system for adsorbing and removing carbondioxide from a gas to be processed in a continuous processing method.

[Carbon Dioxide Processing System]

FIG. 1 illustrates a schematic configuration of a carbon dioxideprocessing system 1 adapted to use a carbon dioxide adsorbent. Thecarbon dioxide processing system 1 illustrated in FIG. 1 is a compositesystem employing a continuous processing method including a carbondioxide separation system 1A configured to selectively separate carbondioxide contained in a gas to be processed by using an adsorbent and acarbon dioxide recovery system 1B configured to desorb (separate) andcollect carbon dioxide from the adsorbent.

The carbon dioxide processing system 1 includes an adsorption vessel 11,a desorption vessel 12, a dryer vessel 13, and a conveyer 15 configuredto convey the adsorbent from an exit port of the dryer vessel 13 to aninlet port of the adsorption vessel 11. The adsorption vessel 11, thedesorption vessel 12, and the dryer vessel 13 are arranged in thissequence from above in the vertical direction to allow the adsorbent tomove from the adsorption vessel 11 to the dryer vessel 13 by gravity.

The adsorption vessel 11 receives a supply of the adsorbent conveyed bythe conveyer 15 from an inlet port provided on an upper portion at apredetermined supply rate. The adsorbent is ejected from an exit portprovided at a lower portion of the adsorption vessel 11 at apredetermined ejection speed.

A gas to be processed generated by a gas-to-be-processed source 35 isintroduced to the lower portion of the adsorption vessel 11 through agas-to-be-processed supply pipe 36. The gas to be processed contains 10to 30% of carbon dioxide such as a combustion exhaust gas at a pressureclose to an ordinary pressure. The gas-to-be-processed supply pipe 36may be provided with at least one pre-processing vessel 37. In thepre-processing vessel 37, the gas to be processed is cooled down to anadequate temperature for adsorption action of carbon dioxide. The gas tobe processed introduced into the adsorption vessel 11 may be subjectedto preprocessing such as desulfurization, dedusting, temperaturedecrease, and dehumidification in addition to cooling.

In the adsorption vessel 11, a moving bed is formed, in which the gas tobe processed flowing upward and the adsorbent flowing downward come intocontact with each other. The adsorbent coming into contact with the gasto be processed selectively adsorbs carbon dioxide contained in the gasto be processed. The temperature of the adsorbent at the time ofadsorption is, for example, 40° C. The gas to be processed (off gas)free from carbon dioxide after separation and removal of carbon dioxideis discharged from the upper portion of the adsorption vessel 11. Incontrast, the adsorbent that has adsorbed carbon dioxide is ejected fromthe lower portion of the adsorption vessel 11 and moves to an inlet portof the desorption vessel 12 under its own weight.

In the desorption vessel 12, the adsorbent that has adsorbed carbondioxide is supplied from an inlet port provided at an upper portion andthe adsorbent is ejected from an exit port provided at a lower portionat a predetermined ejection rate to let the adsorbent move in the vesselfrom the top toward the bottom at a predetermined rate. A lower portionof the desorption vessel 12 receives a supply of desorption vaporsupplied from a vapor generator 38.

In the desorption vessel 12, a moving bed is formed, in which desorptionvapor flowing upward and the adsorbent flowing downward come intocontact with each other. When the desorption vapor comes into contactwith the adsorbent, the desorption vapor is condensed on a surface ofthe adsorbent, and simultaneously emits condensation heat. Carbondioxide is separated from the adsorbent by using the condensation heatas energy for separation.

A carbon dioxide holder 17 is connected to the upper portion of thedesorption vessel 12 via a carbon dioxide recovery pipe 31. The carbondioxide recovery pipe 31 is provided with a pump 16 configured to feed agas in the desorption vessel 12 to the carbon dioxide holder 17. A gasin the desorption vessel 12 (that is, carbon dioxide) is forcedlydischarged to the carbon dioxide recovery pipe 31, is compressed by thepump 16, and is stored in the carbon dioxide holder 17. In contrast, theadsorbent containing condensed water after desorption of carbon dioxideis ejected from the lower portion of the desorption vessel 12 and movesto an inlet port of the dryer vessel 13 under its own weight.

In the dryer vessel 13, the adsorbent containing the condensed water issupplied from an inlet port provided at an upper portion, and theadsorbent is ejected from an exit port provided at a lower portion at apredetermined ejection rate to let the adsorbent move in the vessel fromthe top toward the bottom at a predetermined rate. The adsorbent isdried as it moves in the dryer vessel 13. The adsorbent is dried bycontact between drying gas supplied from the drying gas source 39 to alower portion of the dryer vessel 13 and flowing upward in the vesseland an adsorbent moving downward in the vessel. The drying gas (dryingexhaust gas) used for drying the adsorbent is discharged from the upperportion of the dryer vessel 13

The adsorbent after drying is ejected from the lower portion of thedryer vessel 13, drops onto the conveyer 15, is transferred to theadsorption vessel 11 by the conveyer 15 while being cooled, and isreused as the adsorbent of carbon dioxide.

[Carbon Dioxide Adsorbent]

The adsorbent (carbon dioxide adsorbent) used in the carbon dioxideprocessing system 1 is formed by making silica gel carry an aminecompound.

The amine compound is at least one compound selected from a group ofamines having at least one hydroxyl group and polyamines. In otherwords, the amine compound may contain a mixture of amines andpolyamines. Amines and polyamines of this type are known to reversiblydesorb carbon dioxide, that is, to adsorb and desorb carbon dioxide.Examples of the amine compound including amines having at least onehydroxyl group include monoethanolamine, diethanolamine, andtriethanolamine. Examples of the amine compound including polyaminesinclude polyethyleneimine, ethylenediamine, diethylenetriamine,triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine.

The silica gel described above has particle sizes ranging from 1 mm to 5mm inclusive. The silica gel has pore volumes ranging from 0.1 cm³/g to1.3 cm³/g inclusive. In addition, the average pore diameter of silicagel ranges from 10 nm to 100 nm inclusive. All the particle size, thepore volume, and the average pore diameter described above are values ofsilica gel without carrying the amine compound.

FIG. 2A is a view schematically illustrating the particle size of silicagel, and FIG. 2B is a view schematically illustrating the pore volumeand the pore diameter of silica gel. In FIG. 2B, a cross section ofsilica gel near a surface is illustrated in an enlarged scale. Asillustrated in FIG. 2A, the silica gel is a spherical particle. The“spherical” particle does not necessarily mean an exactly sphericalparticle, but is a particle having a shape that allows more than 90% involume to roll down when placed on a flat and smooth plate inclined byapproximately 30 degrees from a horizontal position.

As described above, carbon dioxide adsorbent formed of silica gel havingparticle sizes ranging from 1 mm to 5 mm inclusive as described above asa base material also have particle sizes ranging from 1 mm to 5 mminclusive. Although silica gel carries amine compound, the aminecompound enters pores formed in the interior of the silica gel.Therefore, the diameter of silica gel is not much increased by carryingthe amine compound.

When the particle size of the carbon dioxide adsorbent is smaller than 1mm, the adsorbent flows with a smaller amount of gas, and thus themoving bed is not formed. In contrast, when the particle size of carbondioxide adsorbent exceeds 5 mm, the weight of the adsorbent increaseswith an increase of particle size. Therefore, the degree of abrasioncaused by an impact subjected when the carbon dioxide adsorbent fallsdown increases, and thus lifetime of the adsorbent is dramaticallyshortened. Therefore, using carbon dioxide adsorbent having particlesizes ranging from 1 mm to 5 mm inclusive facilitates formation of themoving bed to allow an adsorbent and a gas to be processed to come intosuitable counterflow contact with each other, and the adsorbent can beprovided with a suitable life time.

As used herein, the term the “particle size” of carbon dioxide andsilica gel is intended to mean the particle diameter. The particle sizesof carbon dioxide and the silica gel can be measured by following steps(1) to (4), for example.

(1) 100 pieces or more of silica gel sample are arranged on a black feltin such a manner that mutual contact of the particles is avoided as muchas possible.

(2) An image of particles of the silica gel is shot with a view range of100 mm×140 mm.

(3) Using an image processing software ImageJ (National Institutes ofHealth, NIH), the image obtained through shooting is binarized to obtaina surface area of each particle.

(4) Assuming that the particle has an exact spherical shape, theparticle size is obtained from the obtained surface area of eachparticle.

Note that from the obtained particle sizes, a number-average diameter(=Σ(particle size)/(evaluated number of particles)) is obtained, and thenumber-average diameter may be used as the particle size.

As illustrated in FIG. 2B, the average pore diameter of silica gel is anaverage value of diameters of pores formed on the surface of silica gel.The pore volume of the silica gel is the capacity of the pores.

The pore volume of silica gel is obtained by the mercury intrusiontechnique. The average pore diameter of silica gel is obtained byobtaining pore diameter distribution by the mercury intrusion technique,and then obtaining an average pore diameter (median diameter) at thetime of intrusion of mercury of 50% of the entire pore volume. Themercury intrusion technique is a technique of applying a pressure forintruding mercury into pores of powder by using a characteristic ofmercury having a high surface tension and obtaining the specific surfaceor the pore distribution from the pressure and the intruded amount ofmercury. For example, a mercury porosimeter (PASCL240) manufactured byThermo Qurest Italia can be used.

The average pore diameter of silica gel significantly affects theadsorption velocity of the adsorbent. The adsorption velocity of theadsorbent depends on the velocity of diffusion of carbon dioxidediffusion in the pore and the velocity of the adsorption reaction of theadsorbent. The velocity of the adsorption reaction is sufficientlyhigher than the velocity of diffusion of carbon dioxide in the pore, andthus the actual adsorption velocity of the adsorbent is regulated by thevelocity of diffusion of carbon dioxide in the pore.

FIG. 3 is graph illustrating a relationship between carbon dioxide-airdiffusion resistance in a porous substance and the pore diameter. FIG. 3shows that the carbon dioxide-air diffusion resistance is abruptly andsignificantly increased when the pore diameter is reduced to a sizesmaller than 10 nm. This is considered to be because the rate of Knudsendiffusion in the total diffusion resistance in the pore becomes 90% orhigher. Therefore, it is considered that achievement of saturationadsorption of carbon dioxide to the adsorbent within the actualprocessing time is difficult when the average pore diameter of silicagel is reduced to a size smaller than approximately 10 nm. In contrast,when the average pore diameter of silica gel exceeds 100 nm, thestrength of bone structure of primary particles of silica constitutingsilica gel is lowered, and thus the particle strength required for theadsorbent is not satisfied.

From these reasons, by using silica gel having an average pore diameterin the range from 10 nm to 100 nm inclusive, the adsorption velocity ofcarbon dioxide of the adsorbent is maintained in a suitable range, andthe particle strength required for the adsorbent is achieved.

The average pore diameter of silica gel may be controlled by applyingsteaming processing to silica gel. More specifically, the average porediameter can be controlled in a range from 10 nm to 100 nm inclusive byadjusting the pressure and pH of xerogel at the time of steaming. Thesteaming process is normally performed by circulating steam in anautoclave under pressurization, and the processing time is normally from10 minutes to 24 hours. The pressure at this time is of 0.5 to 20Kg/cm². In addition, the pH of xerogel during the steaming processing ispreferably controlled within a range from 5 to 9.

FIG. 4 is a graph illustrating a relationship between the pore volume ofsilica gel in an adsorbent composed of silica gel carryingdiethanolamine (concentration of amine solution: 40%) and an amount ofadsorption of carbon dioxide. FIG. 4 shows that adsorption of carbondioxide is enabled with an adsorbent having a pore volume of silica gelof approximately 0.1 cm³/g or larger.

FIG. 4 also shows that, as the pore volume of silica gel becomes larger,the amount of adsorption of carbon dioxide is increased. However, whenthe pore volume of silica gel is excessively increased, the strength ofsilica gel is reduced to a level difficult to maintain the sphericalshape, and consequently, the particle strength required for theadsorbent cannot be satisfied. Accordingly, based on FIG. 4 , an upperlimit of the pore volume of silica gel is suitably defined to 1.3 cm³/g,at which the amount of adsorption of carbon dioxide is saturated.

From these reasons, by using silica gel having a pore volume rangingfrom 0.1 cm³/g to 1.3 cm³/g inclusive, adsorption of carbon dioxide isenabled, and simultaneously, a particle strength required for theadsorbent can be provided. From the viewpoint of maintaining thespherical shape of silica gel, a condition that the specific area rangesfrom 10 m²/g to 300 m²/g inclusive for silica gel having a particle sizeranging from 1 mm to 5 mm inclusive may be imposed. The specific are ofsilica gel can be obtained by mercury intrusion technique in the samemanner as the average pore diameter and the pore volume.

[Method for Manufacturing Carbon Dioxide Adsorbent]

The above-described adsorbent can be manufactured by a method describedbelow.

Firstly, an amine aqueous solution having an amine compoundconcentration ranging from 5% to 70% inclusive and a temperature rangingfrom 10° C. to 100° C. inclusive is prepared.

The viscosity of the amine aqueous solution is known to be lowered withan increase in temperature. Therefore, the temperature of the amineaqueous solution is preferably 10° C. or higher in order to make silicagel uniformly carry amine compound. In contrast, when the temperature ofthe amine aqueous solution exceeds 100° C., the amine compound tends tooxidize or evaporate easily. Therefore, the suitable temperature of theamine aqueous solution ranges from 10° C. to 100° C. inclusive.

The amine compound concentration in the amine aqueous solutionpreferably is an adequate value according to the amine compound in arange from 5 wt % to 70 wt % inclusive. For example, when the aminecompound is diethanolamine, a suitable concentration of the aminecompound in the amine aqueous solution ranges from 5 wt % to 55 wt %inclusive. For example, when the amine compound is polyethyleneimine, aconcentration around 10 wt % is preferable because the polyethyleneimineis high viscous.

When the concentration of the amine compound in the amine aqueoussolution is lower than a lower limit value, the amount of amine compoundcarried by silica gel is not sufficient, and a huge amount of sumpsolution results after carrying processing. In contrast, when theconcentration of the amine compound in the amine aqueous solutionexceeds an upper limit value, the amine compound closes the pores ofsilica gel, and thus lowering of adsorptive performance of the adsorbentmay result. In the range of the concentration of the amine compound from5 wt % to 70 wt %, the amount of carried amine on the silica gelincreases with an increase in concentration of the amine compound in theamine aqueous solution, and the specific gravity of the adsorbent andthe amount of carbon dioxide adsorbed by the adsorbent increase inproportion. From these reasons, by setting the concentration of theamine compound in the amine aqueous solution adequately in a range from5 w % to 70 wt % inclusive, carbon dioxide adsorbent according to adesign specification of the adsorbent can be manufactured.

Subsequently, silica gel is impregnated with the amine aqueous solutionprepared as described above. The duration of impregnation of silica gelis, for example, 24 hours.

Finally, excess of liquid adhered to silica gel is removed by a methodsuch as suction filtration, and then the silica gel carrying the aminecompound is air-dried at a temperature close to the room temperature.The adsorbent can be manufactured in steps described above.

It is known that part of silica gel is subjected to breakage or crackingdue to adsorption of moisture in a process of impregnating silica gelwith amine aqueous solution in the method for manufacturing theadsorbent described above. The adsorbent having no spherical shape dueto breakage or cracking does not have a sufficient strength (abrasionresistance) for being used in the carbon dioxide processing systememploying the continuous processing method, and is not suitable for use.Therefore, only the adsorbent having a spherical shape is selectivelytaken out from the manufactured adsorbent to be used for adsorption ofcarbon dioxide.

In order to improve yield of adsorbent in manufacture, silica gel havinga waterproof property may be used as the silica gel described above. Asused herein, the term “silica gel having waterproof property” is definedas silica gel having a water proof property N defined by the followingexpression (1), and N is not lower than 45%.N=(W/W ₀)×100  (1)where N: waterproof property [%], W₀: total number [pieces] of particlesof silica gel impregnated with water, W: number of silica gel particlesnot being subjected to cracks out of W₀.

The silica gel having the waterproof property as described above may becommercially available waterproof spherical silica. The waterproofspherical silica can be manufactured by sintering spherical silicaxerogel prepared by drying silica hydrogel obtained, for example, byneutralizing alkaline silicate aqueous solution at a temperature rangingfrom 100 to 1000° C. by superheat steam at a temperature ranging from500 to 1000° C.

The present inventors have confirmed that when the silica gel having thewaterproof property as described above is employed as a material, thesilica gel is rarely subjected to breakage or cracks in a process ofimpregnating the silica gel with amine aqueous solution in the methodfor manufacturing the above-described adsorbent.

EXAMPLES

<Procedure of Preparing Sample of Carbon Dioxide Adsorbent>

A sample of carbon dioxide adsorbent was prepared in the procedure from(1) to (5) given below.

(1) Approximately 300 mL of a base material (silica gel) was extractedand the weight (W) was measured.

(2) A drug (amine compound) was diluted to a predetermined concentrationof carried solution (%) and was put into a bottle.

(3) The base material in (1) was put into the bottle of (2), and wasleft standstill at a room temperature for eight hours. Here, the ratioof weight between the base material and the drug was 1:3.

(4) The base material was taken out from (3) and was subjected tocentrifugation (1100 rpm).

(5) The container was filled with the base material and dry gas(nitrogen gas at 40° C. at a flow rate of 15 L/min.) was flowed toaeration-dry the base material carrying the drug. Note that two hoursafter the time point when the gas temperature at the exit port of thedryer vessel for drying the base material that carries the drug was setto the condition for terminating the drying operation for the materialcarrying the drug (that is, carbon dioxide adsorbent).

<Carbon Dioxide Adsorption Test>

In order to evaluate the carbon dioxide adsorptive performance of acarbon dioxide adsorbent, a carbon dioxide adsorption test (column test)was conducted for the carbon dioxide adsorbent sample in the followingprocedure from (1) to (4).

(1) An adsorption vessel having an inner diameter of 25 mm was filledwith a sample to a height of 300 mm. The amount of filled sample was 147mL.

(2) The adsorptive gas was flowed into the bypass line and confirmedthat the concentration was 10.0%. The adsorptive gas contained carbondioxide of 10 volume %, having a moisture of 5% RH or less and atemperature of 25° C. The superficial velocity of the adsorptive gas was0.05 m/s.

(3) The adsorptive gas was flowed into the adsorption vessel, and thegas concentration at the exit port of the adsorption vessel wasmeasured.

(4) The fact that the concentration of the gas at the exit port reached10.0% (that is, the saturated adsorption) was confirmed and theadsorption step was terminated.

<Carbon Dioxide Adsorption Test Result 1>

Samples 1 to 5 were prepared from a base material common in physicalproperty but different in drug carrying conditions (concentration ofcarried solution and type of drug) according to the above-describedsample preparation procedure. The physical property of the base material(silica gel) and the drug carrying conditions of the respective samplesare shown in the following Table 1.

TABLE 1 Pore Pore Average Concentration of Amount of CO₂ diameter VolumeParticle Size Carried Solution adsorption Sample (nm) (ml/g) (mm) DrugType (%) (kg · CO₂/m³) 1 30 1.0 2.7 DEA 40 34.0 2 30 1.0 2.7 DEA 60 37.33 30 1.0 2.7 PEHA 20 50.9 4 30 1.0 2.7 PEHA 40 36.2 5 30 1.0 2.7 TEPA 4049.3 DEA: diethanolamine PEHA: pentaethylenehexamine TEPA:tetraethylenepentamine

An amine compound used in Sample 1 and Sample 2 was diethanolamine(DEA), and an amine compound used in Sample 3 and Sample 4 waspentaethylenehexamine (PEHA), and an amine compound used in Sample 5 wastetraethylenepentamine (TEPA). The concentration of carried solution ofthe drug in Sample 1, Sample 4, and Sample 5 was 40%, the concentrationof carried solution of Sample 2 was 60%, and the concentration ofcarried solution of Sample 3 was 20%.

The adsorption test results of the above-described Samples 1 to 5 wereas follows. The amount of carbon dioxide adsorbed by Sample 1 was 35.4kgCO₂/m³, the amount of carbon dioxide adsorbed by Sample 2 was 52.1kgCO₂/m³, the amount of carbon dioxide adsorbed by Sample 3 was 33.1kgCO₂/m³, the amount of carbon dioxide adsorbed by Sample 4 was 58.4kgCO₂/m³, and the amount of carbon dioxide adsorbed by Sample 5 was 65.9kgCO₂/m³. All the amounts of carbon dioxide adsorbed by these sampleswere 30 kgCO₂/m³ or more. From this result, the carbon dioxideadsorptive performance of these samples can be evaluated objectively tobe good.

From these results described above, Samples 1 to 5 were recognized tohave superior adsorptive property. No breakage of carbon dioxideadsorbent was found visually in Samples 1 to 5.

<Carbon Dioxide Adsorption Test Result 2>

Samples 6 to 15 were prepared under the same drug carrying conditions(the concentration of carried solution was 40 or 60%, the type of drugwas diethanolamine (DEA)), from base materials different in physicalproperty according to the above-described sample preparation procedure.The physical properties of the base material (silica gel) and the drugcarrying conditions of Samples 6 to 15 are shown in the following Table2.

TABLE 2 Pore Pore Average Concentration of Amount of CO₂ diameter VolumeParticle Size Carried Solution adsorption Sample (nm) (ml/g) (mm) DrugType (%) (kg · CO₂/m³) 6 10 1.0 2.7 DEA 40 34.0 7 30 1.0 1.6 DEA 40 37.38 30 1.0 1.6 DEA 60 50.9 9 70 1.1 2.7 DEA 40 36.2 10 70 1.1 2.7 DEA 6049.3 11 30 1.3 2.7 DEA 40 35.9 12 30 1.3 2.7 DEA 60 47.2 13 70 1.3 2.7DEA 40 39.5 14 70 1.3 2.7 DEA 60 56.3 15 80 1.0 2.7 DEA 40 34.4

Sample 6 was prepared by making a base material having a pore diameterof 10 mm, a pore volume of 1.0 ml/g, and an average particle size of 2.7mm carry DEA having a concentration of carried solution of 40%. Sample 7was prepared by making a base material having a pore diameter of 30 mm,a pore volume of 1.0 ml/g, and an average particle size of 1.6 mm carryDEA having a concentration of carried solution of 40%. Sample 8 wasprepared by making a base material having a pore diameter of 30 mm, apore volume of 1.0 ml/g, and an average particle size of 1.6 mm carryDEA having a concentration of carried solution of 60%. Sample 9 wasprepared by making a base material having a pore diameter of 70 mm, apore volume of 1.1 ml/g, and an average particle size of 2.7 mm carryDEA having a concentration of carried solution of 40%. Sample 10 wasprepared by making a base material having a pore diameter of 70 mm, apore volume of 1.1 ml/g, and an average particle size of 2.7 mm carryDEA having a concentration of carried solution of 60%. Sample 11 wasprepared by making a base material having a pore diameter of 30 mm, apore volume of 1.3 ml/g, and an average particle size of 2.7 mm carryDEA having a concentration of carried solution of 40%. Sample 12 wasprepared by making a base material having a pore diameter of 30 mm, apore volume of 1.3 ml/g, and an average particle size of 2.7 mm carryDEA having a concentration of carried solution of 60%. Sample 13 wasprepared by making a base material having a pore diameter of 70 mm, apore volume of 1.3 ml/g, and an average particle size of 2.7 mm carryDEA having a concentration of carried solution of 40%. Sample 14 wasprepared by making a base material having a pore diameter of 70 mm, apore volume of 1.3 ml/g, and an average particle size of 2.7 mm carryDEA having a concentration of carried solution of 60%. Sample 15 wasprepared by making a base material having a pore diameter of 80 mm, apore volume of 1.0 ml/g, and an average particle size of 2.7 mm carryDEA having a concentration of carried solution of 40%.

According to the adsorption test results for the above-described Samples6 to 15, the amounts of carbon dioxide adsorbed by Samples 6, 7, 9, 11,13, and 15 ranged from 34.0 to 39.9 kgCO₂/m³, the amount of carbondioxide adsorbed by Samples 10 and 12 ranged from 40.0 to 49.9 kgCO₂/m³,and the amount of carbon dioxide adsorbed by Samples 3 and 14 rangedfrom 50.0 to 59.9 kgCo₂/m³. In other words, all of Samples 6 to 15 werefound to have superior carbon dioxide adsorptive performance.

From the results described thus far, carbon dioxide adsorbent preparedby making a base material (silica gel) having a particle size rangingfrom 1 mm to 5 mm inclusive, an average pore diameter ranging from 10 nmto 100 nm inclusive, and a pore volume ranging from 0.1 cm³/g to 1.3cm³/g inclusive carry an amine compound was found to have superiorcarbon dioxide adsorptive performance.

<Rotation Abrasion Test>

In order to evaluate abrasion resistance of carbon dioxide adsorbent, arotation abrasion test was conducted in the following procedures (1) to(2).

(1) 245 ml of a sample was put in a cylindrical drum having a singlepartition wall, and the drum was rotated at 60 rpm for 48 hours.

(2) A powdering rate was calculated from a change in weight of thesample between before and after the rotation of the drum.

Detailed procedure of the test other than the conditions described abovewere compliant with JIS K1150; 1994 “5.9 particle strength—5.9.1 a caseof spalled particles having a lowest grain size distribution limit of1.4 mm or more”.

Empirically known in the rotation abrasion test is that if the powderingrate of the sample was equal to or lower than “5 wt %”, a carbon dioxideadsorbent prepared from the corresponding sample according to the carbondioxide adsorbent sample preparation procedure can endure the usage inthe carbon dioxide separation system of the continuous processing methodhaving a moving bed in the apparatus. Therefore, a carbon dioxideadsorbent prepared from a sample having a powdering rate of 5 wt % orlower was evaluated to have abrasion resistance that can endure theusage in the carbon dioxide processing system employing the continuousprocessing method.

FIG. 5 is a graph of a result of a rotation abrasion test conducted onsilica gel. As is clear from the result of the rotation abrasion test,all the samples (silica gel) having an average particle size rangingfrom approximately 1 mm to 5 mm inclusive, a pore diameter ranging fromapproximately 10 nm to 100 nm inclusive, and a pore volume ranging fromapproximately 0.1 cm³/g to 1.3 cm³/g inclusive had powdering rates nothigher than 5 wt %. Therefore, the carbon dioxide adsorbent prepared bymaking silica gel having a particle size ranging from 1 mm to 5 mminclusive, an average pore diameter ranging from 10 nm to 100 nminclusive, and a pore volume ranging from 0.1 cm³/g to 1.3 cm³/ginclusive carry an amine compound has abrasion resistance that canendure the usage in the carbon dioxide separation system of thecontinuous processing method.

The invention claimed is:
 1. A carbon dioxide adsorbent comprising: silica gel, and an amine compound that is carried by the silica gel, wherein the silica gel has: (i) a spherical shape, (ii) a particle size ranging from 1 mm to 5 mm inclusive, (iii) an average pore diameter ranging from 10 nm to 100 nm inclusive, (iv) a pore volume ranging from 0.1 cm³/g to 1.3 cm³/g inclusive, and (v) a waterproof property N that is defined by an expression (1) and that is not lower than 45%, N=(W/W ₀)×100  (1) where N is the waterproof property in percentage (%) of the silica gel, W₀ is a total number of particles of the silica gel immersed in water, W is a number of particles of the silica gel not subjected to breakage out of W₀.
 2. The carbon dioxide adsorbent according to claim 1, wherein the amine compound is at least one compound selected from a group of amines having at least one hydroxyl group and polyamines.
 3. The carbon dioxide adsorbent according to claim 1, wherein the silica gel has an average pore diameter in a range of from 10 nm to less than 100 nm, and the amine compound is an amine compound that reversibly desorbs carbon dioxide.
 4. The carbon dioxide adsorbent according to claim 1, wherein the silica gel has an average pore diameter in a range of from 10 nm to 80 nm.
 5. A carbon dioxide processing system comprising an adsorption vessel including a moving bed, the moving bed being formed in an interior of the adsorption vessel by the carbon dioxide adsorbent according to claim 1, wherein a gas-to-be-processed supply port is formed at a lower portion of the adsorption vessel and configured to receive a supply of a gas to be processed containing carbon dioxide, and an off-gas discharge port is formed at an upper portion of the adsorption vessel and configured to discharge an off gas, the off gas being a gas generated as a result of removal of carbon dioxide through adsorption to the carbon dioxide adsorbent.
 6. The carbon dioxide processing system according to claim 5, wherein the carbon dioxide processing system further comprising a desorption vessel including a moving bed, the moving bed being formed in an interior of the desorption vessel by the carbon dioxide adsorbent ejected from the adsorption vessel, wherein a water vapor supply port is formed at a lower portion of the adsorption vessel and configured to receive a supply of a desorption vapor, and a carbon dioxide discharge port is formed at an upper portion of the desorption vessel and configured to discharge carbon dioxide separated from the carbon dioxide adsorbent. 